1. Field of the Invention
The present invention relates to a semiconductor light emitting element (LED) which is applied to a light source for solid optical writing used in a digital copying machine, a facsimile telegraph, an optical printer, etc., and is also applied to a light source for optical fibers, etc. in optical communication, optical information processing, etc. More particularly, the present invention relates to a semiconductor light emitting element of an end face light emitting type in which the shape of a light emitting portion and a layer structure are improved to control a radiant angle of light.
2. Description of the Related Art
A semiconductor light emitting element of an end face light emitting type is developed. This semiconductor light emitting element of an end face light emitting type is called an end face type LED in the following description. This semiconductor light emitting element has a laminating structure formed on a semiconductor substrate. Light is emitted from an end face approximately perpendicular to a junction portion. The semiconductor light emitting element of an end face light emitting type is applied to a light source for solid optical writing used in a digital copying machine, a facsimile telegraph, an optical printer, etc., and is also applied to a light source for optical fibers, etc. in optical communication, optical information processing, etc.
Light emitted from this end face LED approximately has a radiant angle having a half-value angle of about 120.degree. in accordance with a Lambert's distribution in a direction parallel to a substrate face. Therefore, for example, the end tip of an optical fiber is formed in a spherical shape or light is guided into the optical fiber through a lens so as to couple light of the end face type LED to the optical fiber at a low loss. However, there is a limit in improvement of optical coupling efficiency. When the emitted light is used through a lens system and its utilization efficiency is improved, it is necessary to increase a numerical aperture (NA) of the lens. Accordingly, there are many problems associated with the design of the lens.
In particular, in the case of the end face type LED, an entire radiant angle of light in a direction perpendicular to the substrate face can be easily set to be equal to or smaller than 25.degree. by optimizing a layer structure. Accordingly, it is necessary to reduce the radiant angle of light parallel to the substrate face so as to improve the optical coupling efficiency with respect to the optical fiber and the lens. In contrast to this, in the case of a semiconductor laser, the entire radiant angle of light perpendicular to a substrate face is larger than that parallel to the substrate face so that the relation between these radiant angles is opposite to that in the case of the end face type LED. Therefore, light perpendicular to the substrate face is diaphragmed as an effective means for improving the optical coupling efficiency with respect to the optical fiber and the lens.
In any case, it is necessary to position the light emitting element with respect to an optical element such as a lens with high accuracy when the light emitting element and the optical element are in hybrid combination with each other. Accordingly, reproducibility and yield are reduced and an expensive jig, etc. are required.
A method for monolithically forming a light emitting element of an end face light emitting type and a lens on the same semiconductor substrate is proposed as a method for omitting a positioning operation Of the light emitting element and the lens. For example, this method is shown in a semiconductor laser of a microlens integrated type (see FIG. 9) described in reference "O plus E, page 112, March, 1991". In this semiconductor laser, a lens composed of a quartz material is formed on the same GaAs substrate in front of a light emitting end face of the light emitting element of the end face light emitting type.
This lens is formed as follows. Namely, a semiconductor laminating structure is formed on a semiconductor substrate 1 and includes an active layer 2 for generating light. In this semiconductor laminating structure, an end face for emitting light is formed such that the active layer is exposed in a direction perpendicular to the substrate face. Further, a silicon oxide film, a nitride film, etc. are laminated and formed on the semiconductor substrate in front of this end face. Further, the laminating structure is dry-etched in a lens shape so that a light emitting device monolithically formed by a light emitting element 4 and a lens is completely manufactured.
However, in this case, another material process is newly required after the completion of a semiconductor process for forming the light emitting element. Accordingly, the number of manufacturing processes is increased and the manufacturing processes are complicated. Therefore, there are problems about yield, reproducibility, uniformity in semiconductor array, etc.